Composite structure for electronic microsystems and method for production of said composite structure

ABSTRACT

The invention relates to a composite structure for electronic microsystems and a method for producing this composite structure, with the composite structure being provided with a polycrystalline diamond layer ( 4 ) for heat withdrawal. The growth substrate ( 1 ) contains or forms a component layer ( 2 ) with the electronic microsystems, which are provided with binary or higher order component compound semiconductors. A protective layer ( 3 ), which encloses the component layer at least indirectly almost entirely, is placed between the component layer  2  and the diamond layer ( 4 ). A material is selected for the protective layer whose reactivity with the precursor materials present in the deposition of the diamond layer ( 4 ) by means of CVD, preferably by means of plasma CVD, is smaller than that of the component layer ( 2 ), and said protective layer. In order for the protective layer ( 3 ) to develop sufficient effectivity, it must be applied with an original thickness of at least 20 nm, preferably at least 50 nm and particularly preferred at least 100 nm.

FIELD OF INVENTION

The present invention relates to a composite structure for electronicsystems and a method for production of this composite structure as theyboth are described in the generic printed publication DE 197 18 618 C2on which the present invention is based.

BACKGROUND OF THE INVENTION

DE 197 18 618 C2 describes a composite structure in whichmicroelectronic components, such as transistors, diodes, resistors,capacitors, inductive resistors, etc., or circuits made thereof, such asamplifiers, sensors, emission cathodes for electrons, etc., placed on agrowth substrate of monocrystalline silicon are provided with a diamondthermoconductive layer. The printed publication also discloses acorresponding plasma CVD for applying the diamond layer. Thethermoconductive layer is made of diamond, among other things, becausethis material also possesses good thermoconductivity despite electricinsulation. Such a type of thermoconductive layer made, in particular,of polycrystalline diamond is, therefore, excellently suited for thermalmanagement of microelectronic components.

However, depositing the diamond layer on the components must occur at,for epitaxial growth, low temperatures in order not to impair or evendestroy the components.

Furthermore, despite the thermal expansion differing from that of thecoated materials, adhesion of the diamond layer to the coated surfacehas to be good. Due to these conditions, the epitaxial growth ofaforementioned diamond thermoconductive layer must be conducted withgreat care.

With, in particular, growth substrates and/or components provided withbinary, ternary, quaternary or even higher grade compoundsemiconductors, there is the problem that these materials react verystrongly with the hydrogenous plasma during application of the diamond,thereby destroying and/or even completely removing these materials.

EP 0681 314 A2 describes the production of a composite structure inwhich a crystalline-arranged, i.e. heteroepitactic in relation to thesubstrate, diamond layer is deposited on a suited monocrystalline growthsubstrate, e.g. of Si or GaAs, by means of an intermediate layer with acontinues lattice structure. The goal is a structurally high gradediamond layer that is suited for production of electronic components ona diamond base. The purpose of the substrate is to permit orienteddiamond growth and therefore the substrate contains no electroniccomponents. A lattice constant similar as closely as possible to that ofthe diamond is required for this growth. Lattice mismatching should bereduced by employing a monocrystalline intermediate layer with latticeconstants that lie between the lattice constants of the diamond and thatof the substrate.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to develop a composite structurefor microelectronic components and a method to produce this compositestructure, which at least largely has the already mentioned advantagesof a diamond thermoconductive layer.

The object is solved according to the present invention with a compositestructure with the features as claimed respectively with a method withthe method steps as claimed. Due to the invented application and CVDdeposition in sufficient thickness of the substances defined by theirchemical reactivity in the claims, the components of the hydrogenousplasma no longer reach the compound semiconductors of the componentlayer. The component layer of the present composite structure containselectronic microsystems such as for example ICs, semiconductor lasers,sensors, transistors, diodes etc. and/or components such as amplifiers,sensors, emission cathodes for electrons, etc. made thereof.

Expediently, metallic function layers such as strip conductors, etc.,which should at least partially not be covered by the diamond layer, arealso provided with a protective layer.

Depending on the application respectively based on the background of theto-be-protected materials, nitrides and/or oxides and/or carbides and/oroxynitrides and/or diamond-like carbon, especially silicon nitride,preferably Si₃Ni₄ and/or silicon oxide, preferably SiO₂, and/or siliconcarbide, preferably SiC, and/or silicon oxynitrides have proven to besuited as materials for the protective layer. Moreover, aluminumnitride, preferably AlN, and/or aluminum oxide, preferably Al₂O₃, arealso suited. The protective layer may be monocrystalline,polycrystalline (e.g. not of stoichiometric Si-nitride) or alsoamorphous (e.g. of diamond-like carbon).

In order to ensure with certainty the function of the protective layer,its layer thickness is selected in such a manner that it remainsconnected in a continuous, uninterrupted manner during the entiredeposition process of the diamond layer. This applies especially in thecase of self-consumption by the components of the plasma, in which theprotective layer, although with reduced layer thickness, shouldnonetheless remain connected in a continuous, uninterrupted manner: i.e.the protective layer must enclose the pertinent materials at least untila continuous, uninterrupted diamond layer has been deposited. Despitethe arrangement of such types of protective layers which, due to theirlow thermoconductivity, may negatively influence withdrawing heat fromthe diamond layer, heat withdrawal in the invented composite structuresis still sufficient due to the remaining reduced thickness of theprotective layers following deposition of the diamond layer.

The quality of the layer system composed of the diamond layer and theintermediate layer placed on the component layer for heat withdrawal isgiven by the quality factor. The quality factor of the thermal measurecorresponds to the relationship between the temperature rise without thethermal measure, i.e. without the layer system, to the temperature risewith the layer system. Temperature rise refers to the difference betweenthe peak temperature reached in the component layer during designatedoperation of the electronic components and circuits integrated thereinand the ambient temperature. In the present method, the thickness of thediamond layer and the thickness of the intermediate layer are preferablymeasured in such a manner that the finished composite structure has aquality of 1.5 or more. This quality factor depends, among other things,on the thickness of the intermediate layer. With a given thickness ofthe diamond layer, the thickness of the intermediate layer must bedesigned during deposition in such a manner that the above condition isfulfilled, i.e. the intermediate layer should not exceed a certainthickness in the finished composite structure. This maximum thicknessdepends on the thermoconductivity of the intermediate layer. If poorlythermoconductive silicon nitride is employed as the intermediate layer,its thickness should not be more than 20 nm. With AlN as theintermediate layer, 200 nm is tolerable.

Ideally, the thickness of the intermediate layer is selected prior toapplying the diamond layer in such a manner that it is, in particular inthe case of poor thermoconductivity, only a few atom layers thick afterapplication of the diamond layer.

Further useful embodiments of the invention are set forth in thecorresponding subclaims. Moreover, the invention is made more apparentusing the preferred embodiments depicted in the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a 100-fold magnification of a broken edge of an uncoatedInP growth substrate;

FIG. 2 shows a 10,000-fold magnification of an InP growth substrateprovided with a protective layer and a diamond layer;

FIG. 3 shows a 20,000-fold magnification of the surface of the compositestructure according to FIG. 2:

FIG. 4 shows a 20,000-fold magnification of a GaAs growth substrateprovided with a protective layer and a diamond layer;

FIG. 5 shows a 20,000-fold magnification of the surface of the compositestructure according to FIG. 4; and

FIGS. 6 a–6 d show schematic single steps of the method for producingthe present composite structure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a 100-fold magnification of a broken edge of an uncoatedInP growth substrate that was exposed to the process conditions for onlya few seconds like those occurring during deposition of diamond by meansof a BIAS-aided plasma CVD. It is quite evident that the InP growthsubstrate has been badly impaired. Corresponding tests have shown thatan InP substrate of conventional layer thickness is etched completelythrough within only a few minutes.

FIG. 2 shows a 10,000-fold magnified broken edge of a coated InP growthsubstrate 1 that was coated first with a protective layer Si₃N₄ 3, thenwith the polycrystalline diamond layer 4. This test revealed that thelayer thickness of the protective layer 3 should be at least 20 nmbefore deposition of the diamond layer, preferably at least 50 nm andparticularly preferred at least 100 nm.

FIG. 3 shows a 1:20,000-fold magnification of the diamond surface of thecomposite structure according to claim 2. This figure shows thepolycrystalline character of the diamond layer particularly distinctly.

FIG. 4 shows a 20,000-fold magnification of a broken edge of a coatedGaAs growth substrate 1. The GaAs growth substrate 1 was first coatedwith a protective layer 3 of Si₃N₄ and then with the polycrystallinediamond layer 4. It was again revealed that the layer thickness of theprotective layer 3 should be at least 20 nm before commencing depositionof the diamond layer 4, preferably at least 50 nm and particularlypreferred at least 100 nm.

FIG. 5 shows a 1:20,000-fold magnification of the diamond surface of thecomposite structure according to FIG. 4.

This figure, too, shows the polycrystalline character of the diamondlayer particularly distinctly.

The protective layer and the diamond layer were each produced by meansof a prior art EIAS-aided plasma CVD process. The parameters used in theprocess are listed in the following table.

DC power 2.7 kW Substrate temp. 180–360° C. Pressure 10.0–14.6 slm Gasflow 0.5 mbar Carbon sources CH₄, C₂H₄ Ratio C/H 0.15–0.3% Ratio O/C30–100%

Details concerning a method of depositing a diamond layer can be found,for example, in the publication by M. Seelmann-Eggebert et al.,“Heat-Spreading Diamond Films for GaN-based High-Power TransistorDevices”, Diamond and Related Materials 10, 2001, pp. 744–749. Thecontent of its disclosure is included in the present summary of theinvention.

FIGS. 6 a–6 d show schematically single steps of the method forproducing the present composite structure. First a growth substrate 1provided with a component layer with electronic microsystems is providedfor (FIG. 6 a). The component layer 2 can be made of, for example, GaN,the growth substrate of sapphire. In the next step, a protective layer 3of Si₃N₄ is applied onto the component layer 2, with layer 3 enclosingthe component layer 2 entirely (FIG. 6 b). The magnified detail on theright shows as an example a component 6 of this component layer 2 withthe respective connection metallization 5. Then a polycrystallinediamond layer is deposited on the protective layer 3 by means of plasmaCVD. During the first period of this deposition, still before there is athin, full-surface diamond layer, the protective layer 3 is attacked bythe plasma during application of the diamond and, if need be, reduced inthickness. After a thin, full-surface diamond layer has been deposited,reduction stops (FIG. 6 c), which is depicted exaggeratedly in thefigure. The diamond layer deposition process continues until the desiredlayer thickness has been attained (FIG. 6 d).

Using a mask, for example with the aid of a structured photoresist,during the diamond deposition ensures that the diamond layer does notcover the connection metallization 5 regions. The protective layer 3 isthen removed from these regions so that they are accessible for externalcontacting (cf. magnified detail of FIG. 6 d).

1. A method for producing a composite structure for electronicmicrosystems, comprising: providing a growth substrate including acomponent layer with said electronic Microsystems, said component layerincluding binary or higher order compound semiconductors; applying ontosaid component layer a protective layer which encloses said componentlayer at least indirectly completely; and applying onto said protectivelayer a polycrystalline diamond layer for heat withdrawal by CVD whereinmaterial for said protective layer has chemical reactivity withprecursor materials present during said applying of said diamond layerby said CVD which is lower than that of the component layer, and whereinsaid protective layer is applied with an original layer thickness of atleast 20 nm.
 2. The method according to claim 1, wherein said protectivelayer comprises nitrides and/or oxides and/or carbides and/oroxynitrides and/or diamond-like carbon.
 3. The method according to claim1, wherein said protective layer comprises silicon nitride, and/orsilicon oxide, and/or silicon carbide, and/or silicon oxynitrides. 4.The method according to claim 1, wherein said protective layer comprisesaluminum nitride, and/or aluminum oxide.
 5. The method according toclaim 1, further comprising at least a plurality of metallic functionlayers disposed at least regionwise within said diamond layer and/or atleast cover regionwise said diamond layer.
 6. The method according toone of claims 1 to 5, wherein all surface regions of said componentlayer which come into contact with the precursor materials of saiddiamond layer are covered with said protective layer.
 7. The methodaccording to one of claims 1 to 5, wherein the layer thickness of saidprotective layer is selected in such a manner that after application ofsaid diamond layer, said layer thickness has a value which yields athermal quality factor of said diamond layer and said protective layerof 1.5 or more.
 8. The method according to claim 1, wherein said CVD isplasma CVD.
 9. A composite structure for electronic microsystems,comprising: a growth substrate including a component layer with saidelectronic microsytems, and a polycrystalline diamond layer forwithdrawal of heat, wherein said component layer includes binary orhigher order compound semiconductors; and a protective layer, whichencloses said component layer at least indirectly completely, disposedbetween said component layer and said diamond layer, and a chemicalreactivity of said protective layer with precursor materials presentduring deposition of the diamond layer by CVD which is lower than acorresponding chemical reactivity of said component layer.
 10. Thecomposite structure according to claim 9, wherein said protective layercomprises nitrides and/or oxides and/or carbides and/or oxynitridesand/or diamond-like carbon.
 11. The composite structure according toclaim 9, wherein said protective layer comprises silicon nitride, and/orsilicon oxide, and/or silicon carbide, and/or silicon oxynitrides. 12.The composite structure according to claim 9, wherein said protectivelayer comprises aluminum nitride, and/or aluminum oxide.
 13. Thecomposite structure according to claim 9, wherein at least a pluralityof metallic function layers are disposed at least regionwise within saiddiamond layer and/or at least cover regionwise said diamond layer. 14.The composite structure of claim 13, wherein said metallic functionlayers are strip conductors.
 15. The composite structure according toclaim 9, wherein all surface regions of said component layer which comeinto contact with the precursor materials of said diamond layer arecovered with said protective layer.
 16. The composite structureaccording to one of claims 9 to 15, wherein a layer thickness of saidprotective layer has a value with which a thermal quality factor of saiddiamond layer and protective layer of 1.5 or more is achieved.